The present invention relates to a crystal structure arrangement consisting of electrooptic and compensation crystals for use in a spatial light modulator, wherein the electrooptic crystal faces an eletron beam source formed within an vacuum envelope, electrons emitted from the electron beam source are stored on the surface of the electrooptic crystal so that the refractive index of the electrooptic crystal is changed corresponding to a change in the stored charge, and the distribution of the refractive index change is read out by means of a laser beam or an incandescent light beam.
The principle of operation of the conventional spatial light modulator will now be described.
FIG. 1 shows the basic configuration of a spatial light modulator in accordance with the conventional techniques. While being illuminated by incoherent light, an input pattern image 1 passes through lens 2 and then it is incident on photocathode 4 formed on the inside surface of glass envelope 3 of the spatial light modulator.
Responding to the input pattern image 1, photocathode 4 emits photoelectrons. The photoelectrons pass through electronic acceleration/focusing lens system 5 and then they are incident on a microchannel plate 6, which multiplies the electrons by a factor in the order of thousands.
The multiplied electrons pan through mesh electrode 7 and are stored on the surface of electrooptic crystal plate 8, i.e., LiNbO.sub.3. Transparent conductive film electrode 8a is formed on the other surface of plate 8 to change the refractive index thereof corresponding to the electronic charge image thereon.
When a laser beam emitted from laser beam source 10 passes through half mirror 9 and is incident on electrooptic crystal plate 8, laser beam image 11 (a coherent image) is obtained. Laser beam image 11 can be used to perform optical parallel processing by utilizing the coherent light beam.
FIGS. 2(A) and 2(B) show the line of electric force along the thickness of an electrooptic crystal plate.
If electrooptic crystal plate 8 is rather thick, when a point charge P is formed on a surface of the electrooptic crystal plate in the spatial light modulator, the lines of electric force induced by the point charge spread out widely as indicated by .delta..sub.1 in FIG. 2(A).
If electrooptic crystal plate 8 is rather thin, when point charge P is formed on a surface of the crystal in the spatial light modulator, the lines of electric field induced by the point charge spread out narrowly as indicated by .delta..sub.2 in FIG. 2(B).
The lines of electric force formed along the thickness of the electrooptic crystal plate change the refractive index of electrooptic crystal plate 8 and the change of the refractive index can be used to modulate the light beams emitted from laser beam source 10. Thus, a coherent light image reflected from the surface of the electrooptic crystal plate has higher resolution for a thinner crystal structure.
Assume that electrooptic crystal plate 8 is sliced as thin as possible that it is a flatness of .lambda./10 its sides are parallel with a parrellelism of five seconds or better, and that the electrooptic crystal wafer is built into the spatial light modulator.
When an LiNbO.sub.3 single crystal wafer cut at 55 degrees with respect to the optical axis had a diameter of 25 mm and a thickness of 0.3 mm, resolution of the spatial light modulator built in accordance with the conventional techniques was three line pairs per mm at a modulation factor of 50%.
The electrooptic crystal plate 8 used in the spatial light modulator in accordance with the conventional techniques can preferably be made of a crystalline material from which wafers with large areas can easily be produced, and it should hav a low half-wave voltage and an extremely low photoconductivity. It is preferably made of a material having optical properties which do not change after baking at high temperature during fabrication of the photocathode. Thus, an LiNbO.sub.3 single crystal plate cut at 55 degrees with respect to the optical axis is generally used for fabricating the spatial light modulator.
Resolution of the spatial light modulator in accordance with the conventional techniques, however, is lower than that which can be used to peform the optical parallel processing by utilizing a coherent light beam, and the wafer of electrooptic crystal 8 should be made thinner to improve resolution of the spatial light modulator.
If the wafer of electrooptic crystal 8 is made thinner than the above, however it may be deformed to such an extent that said wafer cannot be used in the spatial light modulator.
As described above, the refractive index distribution in the electrooptic crystal structure can be changed in accordance with the stored charge image formed on the surface of an electrooptic crystal plate. When the light beam is incident on the electrooptic crystal plate after passing through a polarizer, the refractive index distribution can be read out by using the reflected light beam passing through an analyzer and then the image can be projected onto a projection screen. The electrooptic crystal plate can be made of KDP, BSO, BGO or LiNbO.sub.3. LiNbO.sub.3 is the most popular crystal among them. The electrooptic crystal plate is mounted on a support in a vacuum envelope wherein an electron beam source is formed.
KDP tends to be degraded during a temperature rise. It may be degraded during heat treatment for exhausting gases from the parts and envelope when the spatial light modulator is being fabricated.
BSO and BGO have photoconductivity which limits the allowable wavelength range.
An LiNbO.sub.3 crystal has endurance against temperature rise and has no photoconductivity. An LiNbO.sub.3 crystal cut at 55 degrees with respect to the optical axis has a half-wave voltage which is lower than that of an LiNbO.sub.3 crystal cut at any other angle. Thus, an LiNbO.sub.3 crystal cut at 55 degrees with respect to the optical axis is suitable for forming an electron charge image.
An LiNbO.sub.3 crystal cut at 55 degrees with respect to the optical axis has a natural birefringent property which degrades resolution of images due to different wavelengths emitted from the incoherent light when it is illuminated by incoherent light.
An LiNbO.sub.3 single crystal has an excellent electrooptic property. However, in the spatial light modulator in accordance with the conventional techniques, the incident light is separated into the ordinary (o) and the extraordinary (e) rays when passing thorugh the LiNbO.sub.3 crystal because of a natural birefringent property therein, and these rays are modulated at different points within the LiNbO.sub.3 crystal. Resolution cannot be improved by this type of spatial light modulator.
If the input pattern image is read out by utilizing a white light beam, the (o) and (e) rays caused by the natural birefringent property depend on the distributed wavelengths of the white light beam.
A single wavelength is required for a read operation and thus a white light beam cannot be used to improve resolution.
The refractive indices for the (o) and (e) rays may change in different ways when the ambient temperature changes. Thus, the light beam read out of LiNbO.sub.3 crystal is modulated by temperature when the temperature changes.
If any compensation crystal is attached to the electrooptic crystal, the incident light may be reflected from the interface between the compensation and electrooptic crystals and thus resolution may be degraded.